Conventionally, there is known a heat-treating apparatus, for example, as shown in FIG. 6, which is constructed to house a wafer holder 3 in which wafers 4 are disposed one above the other within a furnace tube 2 made of silica glass surrounded by heating a means 1 and which performs various heat treatments, such as oxidation, diffusion, vapor phase deposition and annealing, on the surface area of the wafers 4 placed on the wafer holder 3 while heating and maintaining the wafers 4 within a predetermined temperature range by means of the aforementioned heating means 1.
To minimize the space required for installation of this type of heat-treating apparatus, a vertically-structured heat-treating apparatus furnace is usually used in which the foregoing furnace tube 2 is vertically installed. In such an apparatus, however, to prevent variations in the thickness of thin film to be formed or doped on the surfaces of the wafers 4 and in the diffusate density distribution due to uneven temperature, a heat insulator 5 made of silica glass is provided between a heating area A in the furnace tube 2 and the open end of furnace tube 2 to maintain a uniform temperature in the space where the aforementioned wafers 4 are heat-treated. In addition, the heating means 1, which surrounds the aforementioned furnace tube 2, is located above the heat insulator 5 so that the heat insulator 5 functions as a heat insulating material, thereby preventing heat loss through O-rings 6 provided between the open end of the core tube 2 and another sealing member.
Such an apparatus is constructed so that the wafer holder 3 is placed directly on the aforementioned heat insulator 5. However, the heat insulator 5 is generally formed by vacuum sealing lint-like silica glass wool 5b within a sealed cylindrical body 5a made of silica glass, and consequently the heat insulator has low load resistance properties, thus limiting the number and diameter of wafers which can be disposed thereon.
Yet, since the top surface of the aforementioned heat insulator 5 is always exposed to high temperature due to radiant heat from heating means 1 which surrounds the furnace tube 2 above the insulator, if gas (air) remains in the sealed cylindrical body 5a or if pinholes communicating with outside air occur as a result of subsequent cleaning or the like so that the sealing is lost and cleaning liquid or outside air is allowed to enter, the remaining gas (air) or the air or cleaning liquid which has entered the cylindrical body is rapidly heated, thereby further increasing the internal pressure load by thermal expansion or expansion and vaporization, and thus possibly rupturing and destroying the cylindrical body 5a.
To prevent such destruction, ribs, braces or other reinforcing bars have been provided within the foregoing cylindrical body 5a to increase the pressure resistance. In recent years, however, as the diameter of the wafers 4 has become larger and the core tube 2 has been made larger, the diameter of the foregoing heat insulator 5 has also been made larger, which correspondingly decreases the pressure resistance. Accordingly, the number of the aforementioned reinforcing bars to be provided has to be increased in geometric ratio, which makes it more complicated to manufacture the heat insulator 5 and greatly increases the manufacturing cost. In addition, particularly if the diameter of the aforementioned heat insulator 5 is made as large as about 150 to 200 mm, it would be very difficult to design and manufacture an insulator with a large enough number of reinforcing bars to be sufficient to withstand the pressure in a vacuum state and at high temperatures, and as a result, it is not possible to manufacture a heat insulator 5 with such a large diameter.
As a result, even when the furnace tube 2 is made larger with larger diameter wafers 4, it is not possible to correspondingly enlarge the diameter of the aforementioned heat insulator 5. Consequently, the heat from the heat treating space for the wafers 4 within the furnace tube 2 dissipates through the space between the heat insulator 5 and the internal wall surface of the furnace tube 2, thereby causing the heating temperature in the heat treating area A for the aforementioned wafers 4 to fluctuate, and at the same time, making it a problem to prevent high temperatures from spreading to the seal portion at the open end of the furnace tube 2.
For this reason, the present inventors disclosed a technique as shown in FIG. 5, in which, instead of placing a supporting member 3 which serves as a (wafer) holder directly on the foregoing heat insulator 5, the heat insulator 5 is divided into a plurality of cylindrical sub-members 5a, a cylindrical frame body 24 is provided for integrally housing the thus divided cylindrical sub-members 5a, and the structure is arranged so that the aforementioned supporting member 3 is disposed on the aforementioned frame body 24 (Japanese Patent Application No. 62-203499).
In the foregoing apparatus, however, even if the cylindrical sub-members are housed in the frame body 24, they are constructed to be located within the furnace tube 2 in venting communication with the heating area in the vicinity of the supporting member 3 which serves as (wafer) holder. Yet since the aforementioned heat insulator 5 is divided into a plurality of heat sub-insulators which fill its interior, each having a smaller diameter, the function (heat retaining power and heat insulating power) of the heat insulating member itself is adversely affected, and the overall surface area becomes larger. In an apparatus like a CVD apparatus or the like in which vapor phase deposition is carried out in a near vacuum state, for example, gas or particles adsorbed on the surface are scattered during the foregoing vacuum treatment, thus causing the surface of the treated substrate, such as wafer, to be contaminated.
Also, in a preferred embodiment of the foregoing apparatus, hard porous bodies 31 made of silica glass which has a large number of microspaces 31a inside as shown in FIGS. 4(A), (B) and (C), such as sintered silica glass and foamed glass bodies, are used the foregoing heat insulator 5. If, however, the heat insulator is used in the foregoing vacuum or reduced-pressure processing apparatus, a structure is preferred in which the porous body 31 is first subjected to reduced pressure to evacuate it, and then the surface of the porous body 31 is covered and sealed with a transparent glass layer 32 to prevent rupture or damage. However, it is quite difficult to evacuate a porous member existing as an isolated cell or an interconnected cell, and the larger the diameter of the foregoing heat insulating member 5, the more difficult the evacuation is and the longer it takes.
Furthermore, when the heat insulator 5 is constructed to be vented directly to a treatment area for a substrate such as a wafer as mentioned above, a reaction film from the treating gas adheres to the surface of the heat insulating member. To remove the film after the heat treatment, it is necessary to etch and clean the heat insulator 5 with hydrofluoric acid or the like. When the aforementioned glass layer is etched by etch-cleaning, the cleaning liquid may penetrate the interior of the air bubble.
In view of the defects of the prior art, the present invention has been designed, and it is the object of the invention to provide a vertical heat-treating apparatus and a heat insulator with heat insulating and heat retaining properties which have high load resistance and pressure resistance properties capable of processing larger diameter wafers and an increased number of the wafers, which can advantageously be used particularly as a semiconductor, liquid crystal and TFT substrate heat-treating apparatus.
It is another object of the present invention to provide a stable heat insulator capable of being used over a long period of time without the occurrence of any micro-cracking and crazing and further production of peeled substrates, and without collapsing due to progressive cracking even if heat treatment and cooling are repeated many times.